Examples:
To produce a full color palette use:
set palette model HSV functions gray, 1, 1
A nice black to gold palette:
set palette model XYZ functions gray**0.35, gray**0.5, gray**0.8
A gamma-corrected black and white palette
gamma = 2.2
color(gray) = gray**(1./gamma)
set palette model RGB functions color(gray), color(gray), color(gray)